There will be have to be some necessary differences in mindset between
urban and suburban churches because context always affects us deeply.
Our own daughter churches in the NYC suburbs have the same theological
vision and love of the city, but they simply aren’t a) as multi-ethnic
and b) as close to the poor–because the zoning laws of the suburbs tend
to homogenize things economically and therefore, to some degree,
racially. So it is just harder to show how the gospel brings down
racial and class barriers in the suburbs. (According to Ephesians 2,
that is a major sign of the truth of the gospel.) It doesn’t mean that
suburban churches are ‘inferior’ or that it is easier to be a pastor in
the suburbs–I actually think it will take more ingenuity and creativity
to demonstrate the power of the gospel in the suburbs than it will in
the city.

As a pastor in a suburb of Chicago I completely agree. I'm spending this week doing some vision work and asking the same question Daniel did to Keller, and the answers aren't easy.